1. Backgrounds. Their removal and avoidance Tom Shutt Princeton University

2. Background overview Goal: 10 -5 cnt/kg/keV/day after rejection (of 200), except PMTs. Shield: standard, except for use of 5 cm active Xe veto. Neutron moderator Pb (if needed) Cu Xe veto Readout Xe itself mostly pure, except Kr. Key issue: radioactivity of readout.

3. Neutrons Soft neutrons from muons - not important if deep. From rock. Readily reduced by ≈ 20 cm moderator. Very high energy neutrons from muons. Needs further study. Depends on site. From U/Th inside shield.

4. PMTs Standard PMTs very hot. –Example: Borexino 8” Ø “ultra-low-background” PMTs: U 1.4 Bq; Th 0.2 Bq; K 1.9 Bq –Problems: Glass, Ceramic, Dynodes, Components New PMTs. Quartz windows, metal cans. –Hammamatsu R7281Q (reported by S. Moriyama at XENON01 workshop) Shipping part: 4.5 (tube) + 1.6 (base) mBq Nearly a 1000 fold-improvement! Q: survival at low temp? –Burle - microchannelplate based PMT. Developing low background, low temp PMT with Columbia, Yale (->Princeton). Goal: Cu+sapphire+≈1g glass MCP. Have first samples for measurement.

5. Monte Carlo Monte Carlo of 1 MeV events 5 above liquid surface. Liquid: 30 cm Ø, 30 cm deep (65 kg) with 1 cm Cu cryostat. Single E deposition inside Xe, 0-50 keV Locationcnts/kg/keV per original gamma all2.4e-6 < 5 cm deep5.5e-7 < 10 cm deep2.3e-7 With 20 PMTs, each at 6 mBq, 5 cm fiducial background after discrimination = 2 x 10 -5 cnts/kg/keV/day.

7. GEMs, LAAPDs Both much cleaner than PMTs. –Avalanche photodiodes likely very clean. –Kapton in GEMs probably has potassium at ≈hundreds of ppb level. But very low mass Typical values, rate per area: PMT / Cu 5x10 4 PMT / Kapton> 10 5 LAAAPD> 10 5 However - need readout –Separated, cooled SiJFET –Used extensively for CDMS, ≈ 20 cm from detectors.

8. Radioactive impurities in Xe. No long lived Xe isotopes. 85 Kr. –t 1/2 =10.7y,   678 KeV. –Commercial research grade Xe: 10 ppm Kr -> ≈ 200 cnts/kg/keV/day Need 0.1 ppb for 1x10 -5 cnts/kg/keV/day (after discrimination). 42 Ar. More readily removed than Kr. Rn. From emanation from compents, welds. –Typical values: ≤ 0.1 mBq -> ≈ 10 -4 cnts/kg/keV/day –Pprobably adsorbed on cryostat surfaces. U, Th, K. –Very low solubility for ionic impurities in Xe. –Particulates removed by filtering.

9. Removing Kr (+Ar) Two obvious methods: –Distillation –Adsorption Note: both used/developed at Princeton for Borexino. Have a gas sampling mass spectrometer, ≈ 10 -8 sensitivity. – 83 Kr can be measured at needed final level with preconcentration. – Ar measurable directly.

10. Distillation Kr vapor pressure only ≈ 10 larger than Xe Minimum Xe vapor pressure near 1 atm! Column probably large Vapor Pressure

11. Adsorption J.R. Michaels and N.R. Morton., 12th AEC Air Cleaning Conference, 1972. 195 K Adsorption constant vs Temp. Ratio > 100 Chromatographic separation: –Warning: illustrative calculation. –Need clean carrier gas. –Must avoid spread of front Real world ≠ ideal.